The stress will begin in college. Whether it is a Ph.D. or MD that you are pursuing, you will be busting your hump for at least eight years. Papers, presentations, and failed hypotheses will test your patience and pride.
Being around germs and disease all day can be taxing. This is not a job for the weak of stomach. It is also stressful to be surrounded by sickness all the time. Just remember, you got into this profession to help find cures.